The present invention relates to aqueous polymer dispersions in which the polymer particles have two different polymer phases P1 and P2 having different theoretical glass transition temperatures Tg(1) and Tg(2).
The invention also relates to a process for preparing such polymer dispersions. The invention further relates to the use of the polymer dispersions as binders in coating compositions. The invention relates additionally to latex paints comprising such polymer dispersions as binders.
Paints are commonly divided into three categories in accordance with their ability to reflect light:
1. flat paints having a specular gloss of less than 15% reflectance,
2. semi-gloss paints having a specular gloss of about 35% to 50% reflectance, and
3. high-gloss paints having a specular gloss of  greater than 70% reflectance,
based in each case on light having a 60xc2x0 angle of incidence.
Solventborne paints can easily be formulated into these three categories. In the case of latex paints, i.e., paints comprising not only a pigment as coloring constituent but also an aqueous, film-forming polymer dispersion as binder, it is difficult to achieve a high specular gloss. The lower gloss of latex paints in comparison to oil-based paints has its origin in the process of film formation. In comparison to the polymer of the oil paints, which is dissolved at the molecular level, latex polymers are usually of higher molecular weight and are present in the form of individual particles. A retarded or greatly restricted flow of the macromolecules during the process of film formation is the result. This, and remanent textures, are the essential reasons why only a low gloss can usually be obtained with latex paints. In pigmented coating compositions based on aqueous polymer dispersions the quality of the coating depends essentially on the ability of the polymer particles, as the coating composition dries, to bind the pigment particles and any filler present and to form a coherent polymeric film. Of course, the higher the proportion of pigments and fillers in the coating composition, the more difficult this process is.
EP-A 429 207 describes aqueous polymer dispersions whose polymer particles have a core-shell structure, the core-forming polymers having a higher glass transition temperature than the polymers which form the shell. The polymer particles have a size in the range from 20 to 70 nm. The polymer dispersions described therein are used to prepare coating compositions with low levels of pigmentation having improved gloss and improved blocking resistance. A disadvantage is the low particle size, which leads to viscosity problems and stability problems during the preparation of the polymer dispersions.
U.S. Pat. No. 5,182,327 describes aqueous polymer dispersions and high-gloss latex paints prepared from them. The average molecular weight of the polymers present in the dispersions is below 150,000. Furthermore, the polymers are functionalized with from 3 to 20% by weight of an olefinic carboxylic acid. The paints exhibit poor blocking resistance, probably on account of the low molecular weight. Furthermore, in the wet state the coatings are sensitive to mechanical influences. Their scrub resistance (abrasion resistance), in particular, leaves something to be desired.
U.S. Pat. No. 5,506,282 describes aqueous coating compositions based on polymer dispersions which contain two different types of polymer particle having different particle diameters. EP-A 466 409 likewise describes a blend of two different aqueous polymer dispersions, in which one of the polymer particle types has a glass transition temperature above room temperature and the other polymer particle type has a glass transition temperature of below 20xc2x0 C. EP-A 761 778 discloses similar coating compositions, the polymer particles in this case having not only a different glass transition temperature but also different particle sizes.
Coating compositions containing different types of polymer particle are, of course, more complex to prepare, since the different types of polymer particle must be prepared in separate polymerization reactions.
It is an object of the present invention to provide aqueous polymer dispersions which are easy to prepare and which in particular in coating compositions ensure high gloss, good mechanical strength and a high blocking resistance of the coating.
We have found that this object is achieved by means of aqueous polymer dispersions in which the polymer particles have a minimum film-forming temperature of below 65xc2x0 C. and contain the two polymer phases P1 and P2 each with different glass transition temperatures Tg(1) and Tg(2), a chain transfer reagent having been used in the preparation of one of the polymer phases.
The present invention accordingly provides aqueous polymer dispersions having a minimum film-forming temperature of below +65xc2x0 C. comprising at least one film-forming polymer in the form of dispersed polymer particles comprising a polymer phase P1 and a different polymer phase P2, the polymer dispersion being obtainable by free-radical aqueous emulsion polymerization comprising the following steps:
i) polymerization of a first monomer charge M1 to give a polymer P1 having a theoretical glass transition temperature Tg(1) (acccording to Fox) and
ii) polymerization of a second monomer charge M2 to give a polymer P2 having a theoretical glass transition temperature Tg(2) (according to Fox) which is different from Tg(1) in the aqueous dispersion of the polymer P1,
at least one chain transfer reagent being used either in the polymerization of the monomer charge M1 or in the polymerization of the monomer charge M2.
In accordance with the invention, the polymer phases P1 and P2 have different glass transition temperatures Tg(1) and Tg(2). The difference between the glass transition temperatures is generally at least 10 K, preferably at least 20 K, in particular at least 40 K. With very particular preference, the difference between the theoretical glass transition temperatures is from 40 to 150 kelvins.
The term theoretical glass transition temperature as used here and below is the glass transition temperature Tg(1) or Tg(2), respectively, as calculated by the method of Fox on the basis of the monomer composition of the monomer charge M1 and of the monomer charge M2. According to Fox (T. G. Fox, Bull. Am. Phys. Soc. (Ser. II) 1, 123 [1956] and Ullmann""s Enzyklopxc3xa4die der technischen Chemie, Weinheim (1980), pp. 17, 18) the glass transition temperature of copolymers at high molecular masses is given in good approximation by       1          T      g        =                    X        1                    T        g        1              +                  X        2                    T        g        2              +          …      ⁢              xe2x80x83            ⁢                        X          n                          T          g          n                    
where x1, X2, . . . , Xn are the mass fractions of the monomers 1, 2, . . . , n and Tg1, Tg2, . . . , Tgn are the glass transition temperatures of the polymers composed in each case of only one of the monomers 1, 2, . . . , n in degrees Kelvin. These are known, for example, from Ullmann""s Encyclopedia of Industrial Chemistry, VCH, Weinheim, Vol. A 21 (1992) p., 169 or from J. Brandrup, E. H. Immergut, Polymer Handbook 3rd ed, J. Wiley, New York 1989.
In accordance with the invention, the monomer charge M2 is preferably chosen such that the theoretical glass transition temperature (according to Fox) of the resulting polymer phase P2 lies above the theoretical glass transition temperature of the polymer P1 prepared first of all. The monomer charge M2 then preferably has a composition leading to a theoretical glass transition temperature Tg(2) of the polymer phase P2 which lies above 30xc2x0 C., preferably above 40xc2x0 C. and, in particular, in the range from 50xc2x0 C. to 120xc2x0 C.
For Tg(2) greater than Tg(1), the monomer charge MI preferably has a monomer composition leading to a theoretical glass transition temperature Tg(1) of the resulting polymer phase P1 which lies in the range from xe2x88x9240xc2x0 to +40xc2x0 C., preferably in the range from xe2x88x9230xc2x0 C. to +30xc2x0 C. and, with very particular preference, in the range from xe2x88x9210xc2x0 C. to +25xc2x0 C.
Where Tg(1) greater than Tg(2), the preferred glass transition temperatures of the polymer phase P1 are subject to what was said above for P2 where Tg(2) greater than Tg(1). The glass transition temperatures of the polymer phase P2 are then subject, accordingly, to what was said above for Tg(1).
In the polymer dispersions of the invention the weight ratio of the polymer phases to one another is in the range from 20:1 to 1:20, preferably from 9:1 to 1:9. In accordance with the invention, preference is given to polymer dispersions in which the fraction of polymer phase having the lower glass transition temperature is predominant. Where P1, as is preferred in accordance with the invention, has the lower glass transition temperature, the ratio P1:P2 is in particular in the range from 1:1 to 5:1 and, with particular preference, in the range from 2:1 to 4:1. The weight ratios of the polymer phases P1 and P2 in that case correspond approximately to the quantitative ratios of the monomer charges M1 and M2.
In the case of Tg(1) greater than Tg(2), the quantitative ratios P1:P2 are in particular in the range from 1:1 to 1:5 and, with particular preference, in the range from. 1:2 to 1:4.
In accordance with the invention, the aqueous polymer dispersions preferably have minimum film-forming temperatures in the range below +65xc2x0 C., in particular below 40xc2x0 C. The minimum film-forming temperature is understood firstly to be the experimentally determinable temperature below which the aqueous polymer dispersion no longer forms a coherent film. The minimum film-forming temperature (MFT) can be determined experimentally down to an MFT of 0xc2x0 C. At lower temperatures, the MFT can be estimated from the glass transition temperatures of the polymer phases P1 and P2, the MFT corresponding approximately to the glass transition temperature of the polymer phase having the lower glass transition temperature, provided this polymer phase predominates. The estimation of minimum film-forming temperatures on the basis of glass transition temperatures is familiar to the skilled worker. The MFT thus estimated of the polymer dispersions of the invention is in general above xe2x88x9235xc2x0 C. Preferably, the minimum film-forming temperature is in the range from xe2x88x9220xc2x0 C. to +40xc2x0 C. and, in particular, from 0xc2x0 C. to 40xc2x0 C., in the case, for example, of binders for latex paints.
In accordance with the invention, at least one chain transfer agent (molecular weight regulator) is used during the polymerization of one of the two monomer charges, M1 or M2. The chain transfer agent lowers the molecular weight of the macromolecules which form during the polymerization of the respective monomer charges. Preferably, the weight-average molecular weight of the macromolecules prepared in the presence of the chain transfer agent is in the range from 20,000 to 200,000 and, in particular, in the range from 30,000 to 100,000 (determined by means of GPC). The weight-average molecular weight of the macromolecules not prepared in the presence of a chain transfer agent, on the other hand, is generally above 800,000 and, in particular, above 1,000,000.
Suitable chain transfer reagents are all chemical compounds having the ability under the conditions of a free-radical aqueous emulsion polymerization to terminate the chain growth reaction, e.g., by transferring hydrogen, while not completely inhibiting the polymerization. Examples of suitable chain transfer agents are aldehydes, such as acrolein and methacrolein, allyl alcohols, silanes, organic halogen compounds such as dichloromethane, dibromomethane, chloroform, bromoform, tetrabromomethane and carbon tetrachloride, aromatic compounds such as xcex1-methylstyrene dimer, triphenylmethane, pentaphenylethane, phenols such as 2,6-di-tert-butylphenol and vinylphenol, benzyl vinyl ethers such as xcex1-benzyloxystyrene, xcex1-benzyloxyacrylonitrile and xcex1-benzyloxyacrylamide, and organic sulfur compounds having at least one thiocarbonyl function or an SH function. Chain transfer reagents with SH function are preferred in accordance with the invention. Examples of suitable SH-functional compounds are mercaptoacetic acid, mercaptoethanol, thioglycolic acid, 2-ethylhexyl thioglycolate, especially aliphatic mercaptans of the formula Rxe2x80x94Sxe2x80x94H, where R is a linear or branched alkyl group having preferably 6 to 18 carbon atoms. Examples of suitable aliphatic mercaptans are n-hexyl mercaptan, n-octyl mercaptan, tert-octyl mercaptan, n-dodecyl mercaptan and n-stearyl mercaptan, especially tert-dodecyl mercaptan.
Examples of compounds having thiocarbonyl function are xanthogenates such as dimethylxanthogen disulfide and diethylxanthogen disulfide, and thiurams such as tetramethylthiuram disulfide, tetraethylthiuram disulfide and tetralmethylthiuram monosulfide.
In general, the chain transfer agent is used in an amount of from 0.1 to 10% by weight, preferably from 0.2 to 5% by weight and, in particular, from 0.3 to 4% by weight, based on the monomers to be polymerized in the respective monomer charge. The chain transfer agent is preferably used in the polymerization of the monomer charge M2, especially if it leads to a polymer phase P2 having a higher glass transition temperature than the polymer phase P1. Where the chain transfer agent is used in preparing the polymer phase having the higher glass transition temperature, i.e., preferably in the polymerization of M2, the amount of chain transfer agent is preferably from 0.2 to 5% by weight, in particular from 0.3 to 3% by weight, based on the overall weight of the monomer charge M2. Where the chain transfer agent is used in preparing the polymer phase P1 having the lower glass transition temperature, its amount is preferably from 0.1 to 4% by weight, in particular from 0.2 to 2.0% by weight, based on the overall weight of the monomer charge M1. The amount of chain transfer agent, based on the overall amount of the monomers M1 and M2, will preferably not exceed 2% by weight and in particular will not exceed 1% by weight.
In general, both the polymer phase P1 and the polymer phase P2 are composed essentiallyxe2x80x94i.e., to the extent of at least 80% by weight, preferably at least 90% by weightxe2x80x94of hydrophobic monomers having a water solubility of  less than 60 g/l (at 25xc2x0 C.). Examples of hydrophobic monomers are vinylaromatic monomers, such as styrene, xcex1-methylstyrene, o-chlorostyrene or vinyltoluenes; vinyl esters of aliphatic C1-C18 monocarboxylic acids, such as vinyl acetate, vinyl propionate, vinyl butyrate, vinyl valerate, vinyl hexanoate, vinyl 2-ethylhexanoate, vinyl decanoate, vinyl pivalate, vinyl laurate, vinyl stearate, and commercial monomers VEOVA(copyright) 5-11 (VEOVA(copyright) X is a trade name of Shell and stands for vinyl esters of xcex1-branched aliphatic carboxylic acids having X carbon atoms, which are also called Versatic(copyright) X acids) and also esters of ethylenically unsaturated C3-C8 monocarboxylic or dicarboxylic acids with C1-C18, preferably C1-C12 and, in particular, C1-C8 alkanols or C5-C8 cycloalkanols. Examples of suitable C1-C18 alkanols are methanol, ethanol, n-propanol, i-propanol, 1-butanol, 2-butanol, isobutanol, tert-butanol, n-hexanol, 2-ethylhexanol, lauryl alcohol and stearyl alcohol. Examples of suitable cycloalkanols are cyclopentanol and cyclohexanol. Preferred hydrophobic monomers are, in particular, the esters of acrylic acid and also the esters of methacrylic acid with C1-C12 alkanols, such as methyl (meth)acrylate, ethyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, 1-hexyl (meth)acrylate, tert-butyl (meth)acrylate and 2-ethylhexyl (meth)acrylate. Also suitable are the esters of fumaric acid and of maleic acid, e.g., dimethyl fumarate, dimethyl maleate or di-n-butyl maleate. Furthermore, in addition to the abovementioned monomers, xcex1,xcex2-monoethylenically unsaturated nitriles such as acrylonitrile or methacrylonitrile are suitable hydrophobic monomers. It is also possible, moreover, to use C4-C8 conjugated dienes, such as 1,3-butadiene, isoprene or chloroprene, xcex1-olefins, such as ethylene, propene and isobutene, and vinyl chloride or vinylidene chloride as hydrophobic comonomers.
In addition to the hydrophobic monomers, the polymer phases P1 and P2 generally also contain, in copolymerized form, functional monomers by means of which the properties of the aqueous polymer dispersions can be modified in a known manner. The modifying monomers firstly include monoethylenically unsaturated monomers having at least one acid group in the molecule, or salts of these monomers, examples being the alkali metal salts or the ammonium salts. Examples of monomers of this kind are monoethylenically unsaturated carboxylic acids having 3 to 8 carbon atoms and one or two acid groups in the molecule, e.g., acrylic acid, methacrylic acid, crotonic acid, vinylacetic acid and itaconic acid, and the monoesters of fumaric acid and of maleic acid with C1-C4 alkanols. The monoethylenically unsaturated monomers having at least one acid group further include monoethylenically unsaturated sulfonic acids, such as vinylsulfonic acid, allylsulfonic acid, methallylsulfonic acid, acryloyloxyethylsulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid, and the sodium salts of said sulfonic acids. The monoethylenically unsaturated monomers having an acid group also include monoethylenically unsaturated phosphonic acids, such as vinyl-, allyl- and methallylphosphonic acid, 2-acryloyloxyethylphosphonic acid and 2-acrylamido-2-methylpropanesulfonic acid, and also the salts, especially the sodium salts, of said phosphonic acids.
Monoethylenically unsaturated monomers having at least one acid group are generally used in amounts  less than 5% by weight, preferably  less than 3% by weight, e.g., in an amount from 0.1 to  less than 3% by weight and, in particular, in an amount of from 1 to 2.5% by weight, based on the overall weight of the monomer charges M1+M2 (and thus also based approximately on the overall weight of the polymer phases P1+P2). Preferably, both the polymer phase P1 and the polymer phase P2 contain in copolymerized form monoethylenically unsaturated monomers having an acid group. The polymer phase P2 preferably contains a larger proportion of such monomers, e.g., at least 1.5 times the amount and, in particular, twice the amount, based on the monomers copolymerized in the respective polymer phases.
In one preferred embodiment of the invention the polymer phases P1 and/or P2 contain in copolymerized form monomers containing urea groups, e.g., N-vinylurea and N-allylurea, and derivatives of imidazolidin-2-one, e.g., N-vinyl- and N-allylimidazolidin-2-one, N-vinyloxyethylimidazolidin-2-one, N-(2-(meth)acrylamidoethyl)imidazolidin-2-one, N-(2-(meth)acryloyloxyethyl)imidazolidin-2-one, N-[2-((meth)acryloyloxyacetamido)ethyl]imidazolidin-2-one etc. Monomers having urea groups are used preferably in amounts of from 0.1 to 10% by weight, in particular from 0.5 to 5% by weight, based on the overall weight of M1 and M2, in preparing the polymer dispersion of the invention. Monomers of this kind improve the wet adhesion of the coating compositions prepared from the polymer dispersions of the invention; that is, the adhesion of the coating in the damp or swollen state.
Furthermore, the polymer phases P1 and/or P2 mast contain in copolymerized form monoethylenically unsaturated, neutral or nonionic monomers whose homopolymers are of relatively high solubility in water or swellability in water. These monomers are copolymerized preferably in amounts of  less than 5% by weight and more preferably  less than 2% by weight, based on the overall weight of the polymer phases P1 and P2. Monomers of this type improve the stability of the polymer dispersions. Examples of such monomers are the amides, the N-alkylolamides or the hydroxyalkyl esters of the abovementioned carboxylic acids, such as acrylamide, methacrylamide, N-methylolacrylamide, N-methylolmethacrylamide, 2-hydroxyethylacrylamide, 2-hydroxyethylmethacrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate and hydroxypropyl methacrylate.
It is also possible to use bifunctional monomers in preparing the polymers P1 and/or P2. These monomers are copolymerized, if desired, in minor amounts, generally from 0.1 to 5% by weight and, in particular, not more than 1% by weight, based on the overall monomer amount. They are preferably monomers having two nonconjugated, ethylenically unsaturated bonds, examples being the diesters of dihydric alcohols with xcex1,xcex2-monoethylenically unsaturated C3-C8 carboxylic acids, e.g., glycol bisacrylate, or esters of xcex1,xcex2-unsaturated carboxylic acids with alkenols, e.g., bicyclodecenyl (meth)acrylate. Preferred polymers include no copolymerized bifunctional monomers.
In general, the polymer phase having the higher theoretical glass transition temperature, i.e., preferably the polymer phase P2, contains at least 60% by weight, and in particular at least 80% by weight, in copolymerized form, of at least one hydrophobic monomer whose homopolymer has a glass transition temperature  greater than 30xc2x0 C., preferably  greater than 50xc2x0 C. Monomers of this kind include vinylaromatic monomers, especially styrene, and also C1-C4 alkyl esters of methacrylic acid, especially methyl methacrylate. Particular preference is given to the aforementioned C1-C4 alkyl esters of methacrylic acid. In one preferred embodiment of the polymer dispersions of the invention, therefore, the polymer phase P2 is composed to the extent of at least 60% by weight and, in particular, at least 80% by weight of the C1-C4 alkyl esters of methacrylic acid. Further suitable monomers for the polymer phase having the higher glass transition temperature, e.g., the polymer phase P2, are of course all other of the abovementioned monomers, examples being hydrophobic monomers whose homopolymers have a glass transition temperature  greater than 30xc2x0 C., and also monoethylenically unsaturated monomers having an acid group and ethylenically unsaturated monomers having urea groups.
In one very particularly preferred embodiment of the present invention, the polymer phase having the higher glass transition temperature, i.e., preferably the polymer phase P2, contains the following monomers in copolymerized form:
from 60 to 99% by weight, in particular form 80 to 98.5% by weight, of at least one C1-C4 alkyl ester of methacrylic acid, especially methyl methacrylate,
from 0.5 to 10% by weight, in particular from 0.5 to 5% by weight, of at least one of the abovementioned monoethylenically unsaturated monocarboxylic acids, especially acrylic acid or methacrylic acid,
from 0.5 to 10% by weight, in particular from 1 to 7% by weight, of at least one monoethylenically unsaturated monomer having urea groups, and if desired
up to 25% by weight of one or more C1-C8 alkyl acrylates.
The monomer phase having the lower theoretical glass transition temperature, i.e., preferably the polymer phase P1, is composed in general of at least 20% by weight and preferably at least 30% by weight, in particular from 30 to 80% by weight and, with particular preference, from 40 to 70% by weight, of at least one monoethylenically unsaturated, hydrophobic monomer whose homopolymer has a glass transition temperature of  less than 20xc2x0 C., in particular  less than 0xc2x0 C. Preferred monomers of this type are the C2-C12 alkyl esters of acrylic acid. In addition, the polymer phase having the lower theoretical glass transition temperature generally includes further copolymerized monomers different than the hydrophobic, monoethylenically unsaturated monomers having a corresponding glass transition temperature of  less than 20xc2x0 C. These include firstly the abovementioned monoethylenically unsaturated hydrophobic monomers having a corresponding glass transition temperature of more than 30xc2x0 C., monoethylenically unsaturated monomers having at least one acid group, and, if desired, further, modifying monomers, e.g., monoethylenically unsaturated monomers having at least one urea group.
In one preferred embodiment the polymer phase having the lower glass transition temperature corresponds to the polymer phase P1. In this case, the monomer charge M1 contains with very particular preference the following monomers in the following amounts:
from 30 to 80% by weight, in particular from 40 to 70% by weight and, with very particular preference, from 50 to 65% by weight, of at least one C1-C10 alkyl ester of acrylic acid,
from 20 to 60% by weight, in particular from 30 to 50% by weight, of at least one further monoethylenically unsaturated, hydrophobic monomer selected from the C1-C4 alkyl esters of methacrylic acid and from vinylaromatic monomers, especially methyl methacrylate and styrene, and
from 0 to 20% by weight, in particular from 1 to 10% by weight, of one or more modifying monomers, in particular at least one monoethylenically unsaturated carboxylic acid in the aforementioned amounts and, if desired, a monomer having a urea group.
It has further proven advantageous if the polymer particles in the binder polymer dispersion have a weight-average polymer particle diameter in the range from 50 to 1000 nm (determined by means of an ultracentrifuge or by photon correlation spectroscopy; on particle size determination by means of ultracentrifuge see, e.g., W. Mxc3xa4chtle, Makromolekulare Chemie, 1984, Vol. 185, 1025-1039, W. Mxc3xa4chtle, Angew. Makromolekulare Chemie, 1988, 162, 35-42). In the case of binder dispersions having high solids contentsxe2x80x94e.g.,  greater than 50% by weightxe2x80x94based on the overall weight of the binder dispersion it is advantageous on viscosity grounds if the weight-average particle diameter of the polymer particles in the dispersion is xe2x89xa7250 nm. The average particle diameter will generally not exceed 1000 nm and preferably will not exceed 600 nm. For high-gloss paints it has proven advantageous if the polymer particle diameter is in the range from 50 to 250, in particular from 80 to 200 nm. The stated particle sizes relate to the d50 values determined by means of light scattering on 0.01% by weight dispersions. The d50 value is the diameter which 50% by weight of the polymer particles exceed and 50% by weight of the polymer particles fall below.
The aqueous polymer dispersions of the invention are prepared by free-radical aqueous emulsion polymerization of the monomer charges M1 and M2 in the presence of at least one free-radical polymerization initiator and, if desired, of a surface-active substance.
In this procedure, an aqueous polymer dispersion of the polymer P1 is first prepared by free-radical emulsion polymerization of the monomer charge M1 in an aqueous polymerization medium. An emulsion polymerization of the monomer charge M2 is then conducted in the resulting dispersion of the polymer P1. This forms an aqueous polymer dispersion whose polymer particles contain both a polymer phase P1 and a polymer phase P2. The aqueous polymerization medium generally contains less than 10% by weight of, preferably less than 5% by weight of, and in particular no, water-miscible organic solvents that do not participate in the polymerization.
Suitable free-radical polymerization initiators are all those capable of triggering a free-radical aqueous emulsion polymerization. They may include both peroxides, e.g., alkali metal peroxodisulfates, and azo compounds. As polymerization initiators it is common to use what are known as redox initiators, which are composed of at least one organic reducing agent and at least one peroxide and/or hydroperoxide, e.g., tert-butyl hydroperoxide with sulfur compounds, e.g., the sodium salt of hydroxymethanesulfinic acid, sodium sulfite, sodium disulfite, sodium thiosulfate or acetone bisulfite adduct, or hydrogen peroxide with ascorbic acid. Use is also made of combined systems containing a small amount of a metal compound which is soluble in the polymerization medium and whose metallic component is able to exist in a plurality of valence states, an example being ascorbic acid/iron(II) sulfate/hydrogen peroxide, where the ascorbic acid is frequently replaced by the sodium salt of hydroxymethanesulfinic acid, acetone bisulfite adduct, sodium sulfite, sodium hydrogen sulfite or sodium bisulfite and the hydrogen peroxide by organic peroxides such as tert-butyl hydroperoxide or alkali metal peroxodisulfates and/or ammonium peroxodisulfate. Likewise preferred initiators are peroxodisulfates, such as sodium peroxodisulfate. The amount of free-radical initiator systems :used, based on the overall amount of the monomers M1+M2 to be polymerized, is preferably from 0.1 to 2% by weight.
Surface-active substances suitable for conducting the emulsion polymerization are the emulsifiers and protective colloids that are normally employed for these purposes. The surface-active substances are usually used in amounts of up to 10% by weight, preferably from 0.5 to 5% by weight and, in particular, from 1.0 to 4% by weight, based on the overall amount of monomers M1+M2 to be polymerized.
Examples of suitable protective colloids are polyvinyl alcohols, starch derivatives and cellulose derivatives, and vinylpyrrolidone copolymers. An exhaustive description of further suitable protective colloids is given in Houben-Weyl, Methoden der organischen Chemie, Volume XIV/1, Makromolekulare Stoffe [Macromolecular Substances], Georg-Thieme-Verlag, Stuttgart 1961, pp. 411-420.
As surface-active substances it is preferred to use exclusively emulsifiers, whose relative molecular weights, unlike those of the protective colloids, are usually below 2000. They can be either anionic or nonionic in nature. The anionic emulsifiers include alkali metal salts and ammonium salts of alkyl sulfates (alkyl: C8-C12), of sulfuric monoesters with ethoxylated alkanols (EO units: 2 to 50, alkyl: C12 to C18) and with ethoxylated alkylphenols (EO units: 3 to 50, alkyl: C4-C9), of alkylsulfonic acids (alkyl: C12-C18) and of alkylarylsulfonic acids (alkyl: C9 to C18), and also compounds of the formula I, 
where R1 and R2 are hydrogen or C4-C24 alkyl, preferably C8-C16 alkyl, but are not both hydrogen simultaneously and X and Y can be alkali metal ions and/or ammonium ions. It is common to use technical-grade mixtures containing a fraction of from 50 to 90% by weight of the monoalkylated product, an example being Dowfax(copyright) 2A1 (R1=C12 alkyl; DOW CHEMICAL). The compounds I are common knowledge, for example from U.S. Pat. No. 4,269,749, and are available commercially.
Examples of suitable nonionic emulsifiers are ethoxylated mono-, di- and trialkylphenols (EO units: 3 to 50, alkyl: C4-C9), ethoxylates of long-chain alkanols (EO units: 3 to 50, alkyl: C8-C36), and polyethylene oxide/polypropylene oxide block copolymers. Preference is given to ethoxylates of long-chain alkanols (alkyl: C10-C22, average degree of ethoxylation: from 3 to 50) and, of these, particular preference to those based on naturally occurring alcohols or oxo alcohols with a linear or branched C12-C18 alkyl radical and a degree of ethoxylation of from 8 to 50. Anionic emulsifiers or combinations of at least one anionic and one nonionic emulsifier are preferred.
Further suitable emulsifiers can be found in Houben-Weyl, Methoden der organischen Chemie, Volume XIV/1, Makromolekulare Stoffe, Georg-Thieme-Verlag, Stuttgart, 1961, pp. 192-208.
The preparation of the polymer phase P1 can be carried out either as a batch process or in a semicontinuous procedure, the latter variant being preferred. In the case of semicontinuous procedures, the major amount, i.e., at least 70%, preferably at least 90%, of the monomers to be polymerized (in this case of the monomers M1 ) is supplied to the polymerization batch continuously, including by a stepped or gradient procedure, under polymerization conditions. This procedure is also known as the monomer feed technique. In this context it has proven advantageous to supply the monomers M1 in the form of an aqueous monomer emulsion. In parallel with the addition of the monomers M1, the polymerization initiator is fed in. One possible procedure is to include a small portion, i.e., preferably not more than 10% by weight, of the monomers M1 to be polymerized in the initial charge to the polymerization reactor and to heat this initial charge to polymerization temperature. At the same time a portion of the polymerization initiator, generally from 0.5 to 20% by weight, in particular about 10% by weight, is added to the still-cold initial charge, or to the initial charge during heating, or to the initial charge which is at polymerization temperature. Subsequently, the remaining amounts of initiator solution and the polymerization initiator at the rate at which it is consumed are added continuously to the polymerization reaction.
The monomer charge M2 is added subsequent to the monomer charge M1; it is even possible to commence the addition of M2 when at least 80%, preferably at least 90% and, in particular, about 95% of the monomer charge M1 have already been supplied to the polymerization reactor. In parallel with the addition of the monomer charge M2, polymerization initiator at the rate at which it is consumed is supplied to the polymerization reaction.
The addition of the chain transfer reagent can be made at the beginning of the addition of the respective monomer charge or, preferably, in parallel with the addition of the respective monomer charge. With particular preference, the chain transfer agent is dispersed in the respective monomer charge by, for example, being dissolved in the monomer phase.
In addition to the above-described seed-free preparation mode, the polymerization of the monomer phase M1 can also be conducted in the presence of a separately prepared seed latex. This procedure is preferred in accordance with the invention and results in effective control of polymer particle formation and thus in a more defined polymer particle size. In the case of the polymerization of the monomers M1 in the presence of a seed latex, preference will be given to operating in accordance with a feed technique where, in general, the seed latex is included in the cold initial charge and, during or after heating to polymerization temperature, a certain amount of the polymerization initiator, generally from 1 to 20%, in particular about 10%, is added, and then: the monomer charge M1 is supplied in the manner described above. The amount of seed latex used in each case depends of course on the desired particle size and is generally in the range from 0.01 to 10% by weight, based on the overall amount of the monomers M1+M2 to be polymerized. In the case of the preparation of the polymers preferred in accordance with the invention having polymer particle sizes in the range from 50 to 250 nm, it is common to use from 0.1 to 5% by weight, in particular from 0.2 to 3% by weight and, with very particular preference, from 0.5 to 2% by weight, based on the overall amount of the monomers M1 +M2 to be polymerized.
Suitable seed latices are known from the prior art (see e.g. EP-A 40419, EP-A 614 922, EP-A 567 812 and literature cited therein and also Encyclopedia of Polymer Science and Technology, Vol. 5, John Wiley and Sons Inc., New York 1966, p. 847). Normally a polystyrene seed will be used, since this is readily available and permits defined control of the polymer particle size.
In general, the polymer particles of the seed latex have an average particle size in the range from 10 to 200 nm; for the preparation of the finely particulate polymers preferred in accordance with the invention, having a particle size of  less than 250 nm, preference is given to those seed latices in which the polymer particles have average particle diameters in the range from 20 to 80 nm. In principle, the composition of the seed latices is arbitrary. For reasons of more ready availability, a polystyrene seed will generally be used.
The pressure and temperature of polymerization are of minor importance. In general, polymerization is conducted at temperatures between room temperature and 120xc2x0 C., preferably at temperatures of from 40 to 95xc2x0 C., and, with particular preference, between 50 and 90xc2x0 C.
Following the actual polymerization reaction, it may be necessary substantially to free the aqueous polymer dispersions of the invention from odorous substances, such as residual monomers and other volatile organic constituents. This can be done in a manner known per se physically, by distillative removal (especially by way of steam distillation) or by stripping with an inert gas. In addition, the residual monomer content can be lowered chemically by means of free-radical postpolymerization, in particular under the action of redox initiator systems as specified, for example, in DE-A 44 35 423. Preferably, the postpolymerization is conducted with a redox initiator system comprising at least one organic peroxide and an organic sulfite.
Preferably, before being used in the formulations of the invention, the dispersions of the copolymer P are adjusted to a pH in the range from 6 to 10, preferably by adding a nonvolatile base, e.g., alkali metal hydroxides or alkaline earth metal hydroxides, or nonvolatile amines.
By the emulsion polymerization route it is possible in principle to obtain dispersions having solids contents of up to about 80% by weight (polymer content, based on the overall weight of the dispersion). On practical grounds, polymer dispersions having solids contents in the range from 40 to 70% by weight are generally preferred for the formulations of the invention. Particular preference is given to dispersions having polymer contents of approximately 50% by weight. Of course, dispersions having lower solids contents are also suitable in principle for use for the formulations of the invention.
The aqueous polymer dispersions of the invention are stable liquid systems. They form films, and can therefore be used as binders for pigmented and/or filled coating compositions. Examples of pigmented coating compositions are sealants, sealing compounds, polymer-modified dispersion plasters, and paints also referred to as latex paints or emulsion paints. The aqueous polymer dispersions of the invention are particularly suitable as binders for high-gloss emulsion paints.
The aqueous polymer dispersions of the invention are used in the coating compositions in the amount required in each case.
To illustrate the abovementioned invention, the composition of a customary emulsion paint is elucidated below.
Emulsion paints contain generally from 30 to 75% by weight and preferably from 40 to 65% by weight of nonvolatile constituents. Nonvolatiles are all constituents of the formulation other than water, but at least the overall amount of binder, filler, pigment, low-volatility solvents. (boiling point above 220xc2x0 C.), e.g., plasticizers, and polymeric auxiliaries of this overall amount, approximately
i) from 3 to 90% by weight, preferably from 10 to 60% by weight, is accounted for by solid binder constituents (=copolymer P)
ii) from 5 to 85% by weight, preferably from 10 to 50% by weight, by at least one inorganic pigment,
iii) from 0 to 85% by weight, preferably from 5 to 60% by weight, by inorganic fillers, and
iv) from 0.1 to 40% by weight, preferably from 0.5 to 20% by weight, by customary auxiliaries.
The polymer dispersions of the invention are suitable with particular preference for preparing high-gloss emulsion paints. These paints are generally characterized by a pigment volume concentration pvc in the range, from 15 to 30. The pigment volume concentration pvc here is 100 times the ratio of the total volume of pigments plus fillers divided by the total volume of pigments, fillers and binder polymers; cf. Ullmann""s Enzyklopadie der technischen Chemie, 4th edition, Volume 15, p. 667.
Examples of typical pigments ii) for the formulations of the invention, especially for emulsion paints, are titanium dioxide, preferably in the rutile form, barium sulfate, zinc oxide, zinc sulfide, basic lead carbonate, antimony trioxide, lithopones (zinc sulfide+barium sulfate). However, the formulations may also include colored pigments, examples being iron oxides, carbon black, graphite, luminescent pigments, zinc yellow, zinc green, ultramarine, manganese black, antimony black, manganese violet, Paris blue or Schweinfurt green. In addition to the inorganic pigments, the formulations of the invention may also include organic color pigments, e.g., sepia, gamboge, Cassel brown, toluidine red, para red, Hansa yellow, indigo, azo dyes, anthraquinonoid and indigoid dyes, and also dioxazine, quinacridone, phthalocyanine, isoindolinone and metal complex pigments.
Suitable fillers iii) include basically aluminosilicates, such as feldspars, silicates, such as kaolin, talc, mica, magnesite, alkaline earth metal carbonates., such as calcium carbonate, in the form, for example, of calcite or chalk, magnesium carbonate, dolomite, alkaline earth metal sulfates, such as calcium sulfate, silicon dioxide, etc. The fillers can be used as individual components. In practice, however, it is established in particular to use filler mixtures, e.g., calcium carbonate/kaolin and calcium carbonate/talc. Dispersion plasters may also include relatively coarse aggregates, such as sands or sandstone granules. In emulsion paints, of course, finely divided fillers are preferred.
In order to increase the hiding power and to save on the use of white pigments, it is common in the preferred emulsion paints to use finely divided fillers (extenders), e.g., finely divided calcium carbonate or mixtures of different calcium carbonates having different particle sizes. To adjust the hiding powder, the shade and the depth of color it is preferred to use blends of color pigments and fillers.
The customary auxiliaries iv) include wetting agents or dispersants, such as sodium, potassium or ammonium polyphosphates, alkali metal salts and ammonium salts of polyacrylic acids and of polymaleic acid, polyphosphonates, such as sodium 1-hydroxyethane-1,1-diphosphonate, and also salts of naphthalenesulfonic acid, especially the sodium salts thereof. The dispersants are generally used in an amount of from 0.1 to 10% by weight based on the overall weight of the emulsion paint.
Furthermore, the auxiliaries iv) may also include thickeners, examples being cellulose derivatives, such as methylcellulose, hydroxyethylcellulose and carboxymethylcellulose; casein, gum arabic, tragacanth gum, starch, sodium alginate, polyvinyl alcohol, polyvinylpyrrolidone, sodium polyacrylates, water-soluble copolymers based on acrylic and methacrylic acid, such as acrylic acid-acrylamide and methacrylic acid-acrylate copolymers, and what are known as associative thickeners, such as styrene-maleic anhydride polymers, or preferably, hydrophobically modified polyether urethanes, as are described, for example, by N. Chen et al. in J. Coatings Techn. Vol 69, No. 867, 1997, p. 73 and by R. D. Hester et al. in J. Coatings Technology, Vol. 69, No. 864, 1997, p. 109, and the disclosure content of which is hereby incorporated fully into the present specification by reference.
The latex paints prepared with the polymer dispersion of the invention exhibit an improved wet abrasion (scrub) resistance and an increased surface gloss relative to the prior art latex paints. Other important product properties, such as blocking resistance and viscosity of the paints, are not adversely affected.